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© 2012 Delmar, Cengage Learning Chapter 31 Oxyfuel Gases and Filler Metals
© 2012 Delmar, Cengage Learning Objectives Explain the chemical reaction that takes place in any oxyfuel flame List the major advantages and disadvantages of the different fuel gases Demonstrate an ability to choose correct filler metals Explain what conditions affect the selection of filler metal
© 2012 Delmar, Cengage Learning Introduction Oxyfuel processes –Consist of a number of separate processes Burn fuel gas with oxygen –Oxyfuel flame was used for fusion welding as early as the first half of the 1800s –Early use of oxygen with hydrogen or acetylene gas often resulted in flashbacks –Welding was dangerous until the development of the torch mixing chamber Gave a more uniform flame
© 2012 Delmar, Cengage Learning Introduction (cont'd.) –Early 1900s: oxyacetylene flame became more popular –Today: oxyacetylene flame is seldom used on metal thicker than 1/16 inch Other process are faster, cleaner, and cause less distortion
© 2012 Delmar, Cengage Learning Uses of the Oxyfuel Flame Increased use of oxyfuel flame –Cutting ferrous metals Cutting torch –Used by hand or machine –Rapidly cuts out steel parts Large number of manufactured items are touched in some way by the oxyfuel flame –Expanding role
© 2012 Delmar, Cengage Learning Characteristics of the Fuel-Gas Flame Flame condition and purity of gas –Affect flame temperature Optical pyrometer –Gives an accurate temperature reading –Where the temperature is measured makes a difference Differences in heat values may also be misleading –Depending on how they are obtained
© 2012 Delmar, Cengage Learning Fuel Gases Most fuel gases used for welding are hydrocarbons –Atoms are bound together tightly to form molecules –Each molecule of a specific gas has the same type, number, and arrangement of atoms Acetylene: two hydrogen and two carbon atoms Propane: eight hydrogen and three carbon atoms
© 2012 Delmar, Cengage Learning FIGURE 31-2 Chemical formulas for two hydrocarbons used as fuel gases. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Fuel Gases (cont'd.) Number of oxygen atoms to completely combust the fuel gas varies –Combustion of acetylene is divided into two separate chemical reactions Primary combustion Secondary combustion –Final products of all clean-burning hydrocarbon flames are the same
© 2012 Delmar, Cengage Learning FIGURE 31-4 Primary flame reaction (with acetylene as the fuel gas). © Cengage Learning 2012
© 2012 Delmar, Cengage Learning FIGURE 31-5 Secondary flame reaction. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Flame Rate of Burning Combustion rate –Speed at which a flame burns –Determined by heat energy required to break the bonds –Higher combustion rate: more prone the mixture is to backfire or flashback
© 2012 Delmar, Cengage Learning Acetylene (C 2 H 2 ) Characteristics –Produced by mixing calcium carbide with water –Colorless, lighter than air, and has a strong garlic smell –Used inside acetylene cylinders to absorb and stabilize the gas –Withdrawal rate of gas should not exceed one- seventh of the total cylinder capacity per hour
© 2012 Delmar, Cengage Learning Heat and Temperature Neutral oxyacetylene flame –Burns at about 5589 degrees Fahrenheit Maximum temperature of a strongly oxidizing flame is about 5615 degrees Fahrenheit Flame burns in two parts –Inner cone and outer envelope High temperature of oxyacetylene flame: Concentrated around the inner cone –More heat is produced in the secondary flame but the temperature is much lower
© 2012 Delmar, Cengage Learning Liquefied Fuel Gases Obtained in individual cylinders or bulk tanks –Pressure in cylinder is not an indication of the level of gas in the tank Results of high withdrawal rates –Drop in pressure –Lowering of cylinder temperature –Possible freezing of the regulator
© 2012 Delmar, Cengage Learning FIGURE The heat absorbed by the liquid propane causes it to change to a gas. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Methylacetylene-Propadiene (MPS) Characteristics –Many in use today as fuel gases Cutting Heating Brazing Metallizing Welding –Mixture of two or more gases –All manufacturers provide MPS gases as liquefied gases in pressurized cylinders
© 2012 Delmar, Cengage Learning Production Approximately twenty-six MPS gases are sold –Mixed by a local supplier as cylinders are filled –Premixed to the supplier Cylinders should be moved enough to remix before use –Piccolo tube improves the mixing of the gas
© 2012 Delmar, Cengage Learning Temperature and Heat MPS gases –Neutral oxyfuel flame About 5301 degrees Fahrenheit –Heat of primary flame About 570 Btu/ft 3 –Much better than acetylene for heating, brazing, and some types of cutting Slower burn rate makes welding difficult
© 2012 Delmar, Cengage Learning MAPP Characteristics –Trade name for stabilized liquefied mixture of methylacetylene and propadiene gases –Oxy MAPP combusts with a high-heat, high- temperature flame –Gases mixed to produce MAPP have the same atomic composition –Oxy MAPP has a neutral flame of 5301 degrees Fahrenheit –MAPP safety advantage: odor
© 2012 Delmar, Cengage Learning FIGURE MAPP gas molecules. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning FIGURE Explosive limits of MAPP gas in air. MAPP Products
© 2012 Delmar, Cengage Learning Propane and Natural Gas Characteristics –Limited use in welding industry –Often used for heating the shop –Both obtained from the petroleum industry –Chemically, propane is C 3 H 8 ; natural gas is mostly methane (CH 4 ) and ethane (C 2 H 6 )
© 2012 Delmar, Cengage Learning Hydrogen Characteristics –Oxyhydrogen produces only a primary combustion flame Flame is almost colorless –Not widely used in welding because of cost –Fastest burning velocity of the fuel gases –Much lighter than air –Low flame temperature restricts oxyhydrogen flame to cutting
© 2012 Delmar, Cengage Learning Filler Metals Divided into groups –Welding: prefix letter R –Brazing: prefix letter B –Buildup, wear resistance surfacing, or both Tubular welding rods –Designated with an RWC prefix Some filler metals are classified both as a braze welding rod and a brazing rod
© 2012 Delmar, Cengage Learning Ferrous Metals Characteristics –Mainly iron –Other elements are added to change: Strength Corrosion resistance Weldability Other physical properties –Specifications and classes have minimum and maximum limits for alloys that are added
© 2012 Delmar, Cengage Learning Ferrous Metals (cont'd.) Physical changes are most often affected by changes in the percentage of alloys of: –Carbon –Silicon –Manganese –Chromium –Vanadium –Nickel –Molybdenum
© 2012 Delmar, Cengage Learning Mild Steel Ferrous metal filler rods –Generally classified by the AWS as: Mild steel Low alloy steel Cast iron –Mild steel and low alloy steel Most frequently gas welded Easily welded without flux –Cast iron and stainless steels Require fluxes and special techniques
© 2012 Delmar, Cengage Learning Cast Iron Characteristics –Cast iron filler rods for gas welding are small, round, or square iron castings –CI stands for cast iron –High-temperature, borax-based flux must be used –Class RCI is lower-strength filler metal –RCI-A welding rods have a higher tensile strength
© 2012 Delmar, Cengage Learning Summary Oxyacetylene welding process –Most desirable in many cases –Wide variety of fuel gases offers the welder unique challenges –Acetylene process is costly Other cheaper fuels do not have all of the characteristics of acetylene –Proper selection of an oxyfuel filler metal is critical
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